#define _CRT_SECURE_NO_WARNINGS 1
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'

void solve() {
	int n, x; cin >> n >> x;
	vector<int> a(n + 1), b(n + 1);
	for (int i = 1; i <= n; i++) cin >> a[i];
	for (int i = 1; i <= n; i++) cin >> b[i];
	int pos = -1, now = -1;
	for (int i = 1; i <= n; i++) {
		if (a[i] + i - 1 <= x && b[i] > now) {
			pos = i;
			now = b[i];
		}
	}
	cout << pos << endl;	
}

int main() {
	ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
	int t; cin >> t;
	while (t--) {
		solve();
	}
	return 0;
}